SCIENCE WATCH

SCIENCE WATCH; EVOLUTIONARY TRAVELS OF KIWIS AND OSTRICHES

Published: December 30, 1986

A LONGSTANDING puzzle in evolution has been the global distribution of large, flightless birds, as well as their small cousin, the New Zealand kiwi. The ostriches of Africa, cassowaries and emus of Australia, and rheas of South America are native to habitats that have been isolated from one another for millions of years.

It is widely believed that the large flightless birds, or ratites, share a common ancestry. All, including the kiwi, bear a flat breastbone lacking the central ridge found in flying birds, and all live on lands that once formed the southern supercontinent, Gondwanaland.

It was assumed that they evolved from an early Gondwanaland inhabitant. While fossils show that they lived for a time in Europe and Asia, it was assumed that they migrated there from the south.

Now Peter Houde, a researcher at the Smithsonian Institution's National Museum of Natural History in Washington, has proposed that the ostrich derived from a flightless, crane-sized bird that lived in the Northern Hemisphere some 50 million years ago and walked to Africa. His hypothesis is based in part on well-preserved skeletons of the fossil, Palaeotis weigelti, found in both East and West Germany.

The evolution of kiwis seems to have diverged from that of ostriches. They retain features typical of even earlier ancestors of the ostrich and may have flown to New Zealand before becoming flightless, Mr. Houde suggested in a recent issue of Nature.

He believes his hypothesis may require revising the timetable whereby animal chemistry has evolved. The timetable is used to assess the relative closeness of animal species,